Recently, as global awareness of environmental preservation has increased, environmentally-friendly vehicles have been actively introduced to the automobile market. Above all, attentions have been drawn to energy-efficient green vehicles, equipped with idle reduction systems, that provide higher fuel efficiency at lower costs than hybrid vehicles.
The idle reduction system is a system that automatically shuts off the engine of a vehicle when the vehicle stops at traffic lights or railroad crossings, and that improves fuel efficiency by about 10%. Further, it is predicted that automobiles practically equipped with the idle reduction mechanism will be increased in order to cope with global warming and emission regulations.
For the idle reduction equipment vehicle that includes the idle reduction system, the engine starts while the electrical system is currently operated, and a voltage drop at the battery occurs due to the generation of the initial inrush current of the cell motor that is used to start the engine.
That is, in the idle reduction system, since the cell motor requires a large amount of current of the battery for starting the engine, a battery voltage may be dropped to cause erroneous operation of the electronic devices.
In order to supply a voltage and a current necessary for the operation of the electronic devices even in the voltage drop condition of the battery, a boost DC-DC converter is required.
Furthermore, in order to increase efficient energy use for an on-vehicle DC-DC converter, it is required that a battery and an alternator (electrical generator) be employed as power supplies. In other words, it is requested that as a power supply, the on-vehicle DC-DC converter employ the alternator when the engine is operated, or employ the battery during the idle reduction period. The alternator performs power generation by employing the revolutions of the drive shaft when the engine is operated, and generates a voltage higher than the voltage of the battery.
FIG. 1 is a circuit diagram for a conventional DC-DC converter.
Referring to FIG. 1, a conventional DC-DC converter 100 is a DC-DC converter that sets either bypass mode in which, when an input voltage VIN is higher than a predetermined voltage, the input voltage VIN is output, unchanged, as an output voltage VO, or sets boost mode in which, when the input voltage VIN is lower than the predetermined voltage, the input voltage VIN is increased to the predetermined voltage level, and the obtained voltage is output as an output voltage VO. That is, the conventional DC-DC converter 100 sets the bypass mode when an alternator is employed as a power supply, or sets the boot mode when a battery is employed as a power supply.
The DC-DC converter that switches between the bypass mode and the boot mode in this manner is described in, for example, patent literature 1.
In the conventional DC-DC converter 100, when the input voltage VIN is higher than a predetermined voltage that is a desired output voltage, a comparator 102 outputs an active-low signal as a reset signal to a latch circuit 112, which then outputs an active-low signal to an NMOS. Sequentially, the NMOS is rendered off to establish an electrical connection between the input terminal and the output terminal, and the input voltage VIN is output unchanged as the output voltage VO.
Furthermore, when the input voltage VIN is lower than the predetermined voltage, the latch circuit 112 outputs to the NMOS a PWM signal having a duty cycle that corresponds to a load RL, and the NMOS performs switching. Since a current having a triangular waveform flows across a coil L, the voltage across a sense resistor RS becomes a voltage with a triangular waveform (a ramp waveform). An error amplifier 103 outputs an error between a voltage FB, obtained by dividing the output voltage VO by using resistors R1 and R2, and a reference voltage V—FB—REF that corresponds to the predetermined voltage, and a phase compensation impedance element Z accumulates the error and generates an error voltage. Further, the phase compensation impedance element Z also performs phase compensation for the feedback loop of the DC-DC converter. The comparator 102 receives the error voltage, and compares this voltage with the triangular wave voltage. Then, the comparator 102 outputs a reset signal, for which the active-low period is consonant with the error voltage, and the latch circuit 112 outputs a PWM signal having a duty cycle that is consonant with the error voltage.